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<a href="mailto:abu@software-lab.de">abu@software-lab.de</a>

<p align=right>
<i>Monk: "If I have nothing in my mind, what shall I do?"</i><br>
<i>Joshu: "Throw it out."</i><br>
<i>Monk: "But if there is nothing, how can I throw it out?"</i><br>
<i>Joshu: "Well, then carry it out."</i><br>
<i>(Zen koan)</i><br>

<h1>PicoLisp Frequently Asked Questions</h1>

<p align=right>(c) Software Lab. Alexander Burger

<p><ul>
<li><a href="#yet">Why did you write yet another Lisp?</a>
<li><a href="#who">Who can use PicoLisp?</a>
<li><a href="#advantages">What are the advantages over other Lisp systems?</a>
<li><a href="#performance">How is the performance compared to other Lisp systems?</a>
<li><a href="#interpreted">What means "interpreted"?</a>
<li><a href="#compiler">Is there (or will be in the future) a compiler available?</a>
<li><a href="#portable">Is it portable?</a>
<li><a href="#webServer">Is PicoLisp a web server?</a>
<li><a href="#lambda">I cannot find the LAMBDA keyword in PicoLisp</a>
<li><a href="#dynamic">Why do you use dynamic variable binding?</a>
<li><a href="#problems">Are there no problems caused by dynamic binding?</a>
<li><a href="#closures">But with dynamic binding I cannot implement closures!</a>
<li><a href="#macros">Do you have macros?</a>
<li><a href="#strings">Why are there no strings?</a>
<li><a href="#arrays">What about arrays?</a>
<li><a href="#floats">How to do floating point arithmetics?</a>
<li><a href="#bind">What happens when I locally bind a symbol which has a function definition?</a>
<li><a href="#hardware">Would it make sense to build PicoLisp in hardware?</a>
<li><a href="#segfault">I get a segfault if I ...</a>
<li><a href="#ask">Where can I ask questions?</a>
</ul>

<p><hr>
<h2><a name="yet">Why did you write yet another Lisp?</a></h2>

<p>Because other Lisps are not the way I'd like them to be. They concentrate on
efficient compilation, and lost the one-to-one relationship of language and
virtual machine of an interpreted system, gave up power and flexibility, and
impose unnecessary limitations on the freedom of the programmer. Other reasons
are the case-insensitivity and complexity of current Lisp systems.


<p><hr>
<h2><a name="who">Who can use PicoLisp?</a></h2>

<p>PicoLisp is for programmers who want to control their programming
environment, at all levels, from the application domain down to the bare metal.
Who want use a transparent and simple - yet universal - programming model, and
want to know exactly what is going on. This is an aspect influenced by
<code>Forth</code>.

<p>It does <i>not</i> pretend to be easy to learn. There are already plenty of
languages that do so. It is not for people who don't care what's under the hood,
who just want to get their application running. They are better served with some
standard, "safe" black-box, which may be easier to learn, and which allegedly
better protects them from their own mistakes.


<p><hr>
<h2><a name="advantages">What are the advantages over other Lisp systems?</a></h2>

<h3>Simplicity</h3>
<p>PicoLisp is easy to understand and adapt. There is no compiler enforcing
special rules, and the interpreter is simple and straightforward. There are only
three data types: Numbers, symbols and lists ("LISP" means "List-, Integer- and
Symbol Processing" after all ;-). The memory footprint is minimal, and the
tarball size of the whole system is just a few hundred kilobytes.

<h3>A Clear Model</h3>
<p>Most other systems define the language, and leave it up to the implementation
to follow the specifications. Therefore, language designers try to be as
abstract and general as possible, leaving many questions and ambiguities to the
users of the language.

<p>PicoLisp does the opposite. Initially, only the single-cell data structure
was defined, and then the structure of numbers, symbols and lists as they are
composed of these cells. Everything else in the whole system follows from these
axioms. This is documented in the chapter about the <a href="ref.html#vm">The
PicoLisp Machine</a> in the reference manual.

<h3>Orthogonality</h3>
<p>There is only one symbolic data type, no distinction (confusion) between
symbols, strings, variables, special variables and identifiers.

<p>Most data-manipulation functions operate on the values of symbols as well as
the CARs of cons pairs:

<pre><code>
: (let (N 7  L (7 7 7)) (inc 'N) (inc (cdr L)) (cons N L))
-> (8 7 8 7)
</code></pre>

<p>There is only a single functional type, no "special forms". As there is no
compiler, functions can be used instead of macros. No special "syntax"
constructs are needed. This allows a completely orthogonal use of functions. For
example, most other Lisps do not allow calls like

<pre><code>
: (mapcar if '(T NIL T NIL) '(1 2 3 4) '(5 6 7 8))
-> (1 6 3 8)
</code></pre>

<p>PicoLisp has no such restrictions. It favors the principle of "Least
Astonishment".

<h3>Object System</h3>
<p>The OOP system is very powerful, because it is fully dynamic, yet extremely
simple:

<p><ul>
<li>In other systems you have to statically declare "slots". In PicoLisp,
classes and objects are completely dynamic, they are created and extended at
runtime. "Slots" don't even exist at creation time. They spring into existence
purely dynamically. You can add any new property or any new method to any single
object, at any time, regardless of its class.

<li>The multiple inheritance is such that not only classes can have several
superclasses, but each individual object can be of more than one class.

<li>Prefix classes can surgically change the inheritance tree for any class or
object. They behave like Mixins in this regard.

<li>Fine-control of inheritance in methods with <code><a
href="refS.html#super">super</a></code> and <code><a
href="refE.html#extra">extra</a></code>.

</ul>

<h3>Pragmatism</h3>
<p>PicoLisp has many practical features not found in other Lisp dialects. Among
them are:

<p><ul>
<li>Auto-quoting of lists when the CAR is a number. Instead of <code>'(1 2
3)</code> you can just write <code>(1 2 3)</code>. This is possible because a
number never makes sense as a function name, and has to be checked at runtime
anyway.

<li>The <code><a href="refQ.html#quote">quote</a></code> function returns all
unevaluated arguments, instead of just the first one. This is both faster
(<code>quote</code> does not have to take the CAR of its argument list) and
smaller (a single cell instead of two). For example, <code>'A</code> expands to
<code>(quote . A)</code> and <code>'(A B C)</code> expands to <code>(quote A B
C)</code>.

<li>The symbol <code><a href="ref.html#atres">@</a></code> is automatically
maintained as a local variable, and set implicitly in certain flow- and
logic-functions. This makes it often unnecessary to allocate and assign local
variables.

<li><a href="tut.html#funio">Functional I/O</a> is more convenient than
explicitly passing around file descriptors.

<li>A well-defined <a href="ref.html#cmp">ordinal relationship</a> between
arbitrary data types facilitates generalized comparing and sorting.

<li>Uniform handling of <code>var</code> locations (i.e. values of symbols and
CARs of cons pairs).

<li>The universality and usefulness of symbol properties is enforced and
extended with implicit and explicit bindings of the symbol <code><a
href="refT.html#This">This</a></code> in combination with the access functions
<code><a href="ref_.html#=:">=:</a></code>, <code><a
href="ref_.html#:">:</a></code> and <code><a href="ref_.html#::">::</a></code>.

<li>A very convenient list-building machinery, using the <code><a
href="refL.html#link">link</a></code>, <code><a
href="refY.html#yoke">yoke</a></code>, <code><a
href="refC.html#chain">chain</a></code> and <code><a
href="refM.html#made">made</a></code> functions in the <code><a
href="refM.html#make">make</a></code> environment.

<li>The syntax of often-used functions is kept non-verbose. For example, instead
of <code>(let ((A 1) (B 2) C 3) ..)</code> you write <code>(let (A 1 B 2 C 3)
..)</code>, or just <code>(let A 1 ..)</code> if there is only a single
variable.

<li>The use of the hash (<code>#</code>) as a comment character is more adequate
today, and allows a clean hash-bang (<code>#!</code>) syntax for stand-alone
scripts.

<li>The interpreter is <a href="ref.html#invoc">invoked</a> with a simple and
flexible syntax, where command line arguments are either files to be interpreted
or functions to be directly executed. With that, many tasks can be performed
without writing a separate <a href="tut.html#script">script</a>.

<li>A sophisticated system of interprocess communication, file locking and
synchronization allows multi-user access to database applications.

<li>A Prolog interpreter is tightly integrated into the language. Prolog
clauses can call Lisp expressions and vice versa, and a self-adjusting
depth-first search predicate <code>select</code> can be used in database
queries.

</ul>

<h3>Persistent Symbols</h3>
<p>Database objects ("external" symbols) are a primary data type in PicoLisp.
They look like normal symbols to the programmer, but are managed in the database
(fetched from, and stored to) automatically by the system. Symbol manipulation
functions like <code>set</code>, <code>put</code> or <code>get</code>, the
garbage collector, and other parts of the interpreter know about them.

<h3>Application Server</h3>
<p>It is a stand-alone system (it does not depend on external programs like
Apache or MySQL) and it provides a "live" user interface on the client side,
with an application server session for each connected client. The GUI layout and
behavior are described with S-expressions, generated dynamically at runtime, and
interact directly with the database structures.

<h3>Localization</h3>
<p>Internal exclusive and full use of UTF-8 encoding, and self-translating <a
href="ref.html#transient-io">transient symbols</a> (strings), make it easy to
write country- and language-independent applications.


<p><hr>
<h2><a name="performance">How is the performance compared to other Lisp systems?</a></h2>

<p>Despite the fact that PicoLisp is an interpreted-only system, the performance
is quite good. Typical Lisp programs operating on list data structures are
executed in (interpreted) PicoLisp at about the same speed as in (compiled)
CMUCL, and about two or three times faster than in CLisp or Scheme48. Programs
with lots of numeric calculations, however, may be slower on a 32-bit system,
due to PicoLisp's somewhat inefficient implementation of numbers. The 64-bit
version improved on that.

<p>But in practice, speed was never a problem, even with the first versions of
PicoLisp in 1988 on a Mac II with a 12 MHz CPU. And certain things are cleaner
and easier to do in plain <code>C</code> or <code>asm</code> anyway. It is very
easy to write <code>C</code> functions in PicoLisp, either in the kernel, as
shared object libraries, or even inline in the Lisp code.

<p>PicoLisp is very space-effective. Other Lisp systems reserve heap space twice
as much as needed, or use rather large internal structures to store cells and
symbols. Each cell or minimal symbol in PicoLisp consists of only two pointers.
No additional tags are stored, because they are implied in the pointer
encodings. No gaps remain in the heap during allocation, as there are only
objects of a single size. As a result, consing and garbage collection are very
fast, and overall performance benefits from a better cache efficiency. Heap and
stack grow automatically, and are limited only by hardware and operating system
constraints.


<p><hr>
<h2><a name="interpreted">What means "interpreted"?</a></h2>

<p>It means to directly execute Lisp data as program code. No transformation to
another representation of the code (e.g. compilation), and no structural
modifications of these data, takes place.

<p>Lisp data are the "real" things, like numbers, symbols and lists, which can
be directly handled by the system. They are <i>not</i> the textual
representation of these structures (which is outside the Lisp realm and taken
care of the <code><a href="refR.html#read">read</a></code>ing and <code><a
href="refP.html#print">print</a></code>ing interfaces).

<p>The following example builds a function and immediately calls it with two
arguments:

<pre><code>
: ((list (list 'X 'Y) (list '* 'X 'Y)) 3 4)
-> 12
</code></pre>

<p>Note that no time is wasted to build up a lexical environment. Variable
bindings take place dynamically during interpretation.

<p>A PicoLisp function is able to inspect or modify itself while it is running
(though this is rarely done in application programming). The following function
modifies itself by incrementing the '0' in its body:

<pre><code>
(de incMe ()
   (do 8
      (printsp 0)
      (inc (cdadr (cdadr incMe))) ) )

: (incMe)
0 1 2 3 4 5 6 7 -> 8
: (incMe)
8 9 10 11 12 13 14 15 -> 16
</code></pre>

<p>Only an interpreted Lisp can fully support such "Equivalence of Code and
Data". If executable pieces of data are used frequently, like in PicoLisp's
dynamically generated GUI, a fast interpreter is preferable over any compiler.


<p><hr>
<h2><a name="compiler">Is there (or will be in the future) a compiler available?</a></h2>

<p>No. That would contradict the idea of PicoLisp's simple virtual machine
structure. A compiler transforms it to another (physical) machine, with the
result that many assumptions about the machine's behavior won't hold any more.
Besides that, PicoLisp primitive functions evaluate their arguments
independently and are not suited for being called from compiled code. Finally,
the gain in execution speed would probably not be worth the effort. Typical
PicoLisp applications often use single-pass code which is loaded, executed and
thrown away; a process that would be considerably slowed down by compilation.


<p><hr>
<h2><a name="portable">Is it portable?</a></h2>

<p>Yes and No. Though we wrote and tested PicoLisp originally only on Linux, it
now also runs on FreeBSD, Mac OS X (Darwin), Cygwin/Win32, and probably other
POSIX systems. The first versions were even fully portable between DOS, SCO-Unix
and Macintosh systems. But today we have Linux. Linux itself is very portable,
and you can get access to a Linux system almost everywhere. So why bother?

<p>The GUI is completely platform independent (Browser), and in the times of
Internet an application <u>server</u> does not really need to be portable.


<p><hr>
<h2><a name="webServer">Is PicoLisp a web server?</a></h2>

<p>Not really, but it evolved a great deal into that direction.

<p>Historically it was the other way round: We had a plain X11 GUI for our
applications, and needed something platform independent. The solution was
obvious: Browsers are installed virtually everywhere. So we developed a protocol
which persuades a browser to function as a GUI front-end to our applications.
This is much simpler than to develop a full-blown web server.


<p><hr>
<h2><a name="lambda">I cannot find the LAMBDA keyword in PicoLisp</a></h2>

<p>Because it isn't there. The reason is that it is redundant; it is equivalent
to the <code>quote</code> function in any aspect, because there's no distinction
between code and data in PicoLisp, and <code>quote</code> returns the whole
(unevaluated) argument list. If you insist on it, you can define your own
<code>lambda</code>:

<pre><code>
: (def 'lambda quote)
-> lambda
: ((lambda (X Y) (+ X Y)) 3 4)
-> 7
: (mapcar (lambda (X) (+ 1 X)) '(1 2 3 4 5))
-> (2 3 4 5 6)
</code></pre>


<p><hr>
<h2><a name="dynamic">Why do you use dynamic variable binding?</a></h2>

<p>Dynamic binding is very powerful, because there is only one single,
dynamically changing environment active all the time. This makes it possible
(e.g. for program snippets, interspersed with application data and/or passed
over the network) to access the whole application context, freely, yet in a
dynamically controlled manner. And (shallow) dynamic binding is the fastest
method for a Lisp interpreter.

<p>Lexical binding is more limited by definition, because each environment is
deliberately restricted to the visible (textual) static scope within its
establishing form. Therefore, most Lisps with lexical binding introduce "special
variables" to support dynamic binding as well, and constructs like
<code>labels</code> to extend the scope of variables beyond a single function.

<p>In PicoLisp, function definitions are normal symbol values. They can be
dynamically rebound like other variables. As a useful real-world example, take
this little gem:

<pre><code>
(de recur recurse
   (run (cdr recurse)) )
</code></pre>

<p>It implements anonymous recursion, by defining <code>recur</code> statically
and <code>recurse</code> dynamically. Usually it is very cumbersome to think up
a name for a function (like the following one) which is used only in a single
place. But with <code>recur</code> and <code>recurse</code> you can simply
write:

<pre><code>
: (mapcar
   '((N)
      (recur (N)
         (if (=0 N)
            1
            (* N (recurse (- N 1))) ) ) )
   (1 2 3 4 5 6 7 8) )
-> (1 2 6 24 120 720 5040 40320)
</code></pre>

<p>Needless to say, the call to <code>recurse</code> does not have to reside in
the same function as the corresponding <code>recur</code>. Can you implement
anonymous recursion so elegantly with lexical binding?


<p><hr>
<h2><a name="problems">Are there no problems caused by dynamic binding?</a></h2>

<p>You mean the <i>funarg</i> problem, or problems that arise when a variable
might be bound to <i>itself</i>? For that reason we have a convention in
PicoLisp to use <a href="ref.html#transient-io">transient symbols</a> (instead
of internal symbols)

<ol>

<li>for all parameters and locals, when functional arguments or executable lists
are passed through the current dynamic bindings

<li>for a parameter or local, when that symbol might possibly be (directly or
indirectly) bound to itself, and the bound symbol's value is accessed in the
dynamic context

</ol>

<p>This is a form of lexical <i>scoping</i> - though we still have dynamic
<i>binding</i> - of symbols, similar to the <code>static</code> keyword in
<code>C</code>.

<p>In fact, these problems are a real threat, and may lead to mysterious bugs
(other Lisps have similar problems, e.g. with symbol capture in macros). They
can be avoided, however, when the above conventions are observed. As an example,
consider a function which doubles the value in a variable:

<pre><code>
(de double (Var)
   (set Var (* 2 (val Var))) )
</code></pre>

<p>This works fine, as long as we call it as <code>(double 'X)</code>, but will
break if we call it as <code>(double 'Var)</code>. Therefore, the correct
implementation of <code>double</code> should be:

<pre><code>
(de double ("Var")
   (set "Var" (* 2 (val "Var"))) )
</code></pre>

<p>If <code>double</code> is defined that way in a separate source file, and/or
isolated via the <code><a href="ref_.html#====">====</a></code> function, then
the symbol <code><u>Var</u></code> is locked into a private lexical context
and cannot conflict with other symbols.

<p>Admittedly, there are two disadvantages with this solution:

<ol>

<li>The rules for when to use transient symbols are a bit complicated. Though it
is safe to use them even when not necessary, it will take more space then and be
more difficult to debug.

<li>The string-like syntax of transient symbols as variables may look strange to
alumni of other languages.

</ol>

Fortunately, these pitfalls do not occur so very often, and seem more likely in
utilities than in production code, so that they can be easily encapsulated.


<p><hr>
<h2><a name="closures">But with dynamic binding I cannot implement closures!</a></h2>

<p>This is not true. Closures are a matter of scope, not of binding.

<p>For a closure it is necessary to build and maintain a separate environment.
In a system with lexical bindings, this has to be done at <i>each</i> function
call, and for compiled code it is the most efficient strategy anyway, because it
is done once by the compiler, and can then be accessed as stack frames at
runtime.

<p>For an interpreter, however, this is quite an overhead. So it should not be
done automatically at each and every function invocation, but only if needed.

<p>You have several options in PicoLisp. For simple cases, you can take
advantage of the static scope of <a href="ref.html#transient-io">transient
symbols</a>. For the general case, PicoLisp has built-in functions like <code><a
href="refB.html#bind">bind</a></code> or <code><a
href="refJ.html#job">job</a></code>, which dynamically manage statically scoped
environments.

<p>Environments are first-class objects in PicoLisp, more flexible than
hard-coded closures, because they can be created and manipulated independently
from the code.

<p>As an example, consider a currying function:

<pre><code>
(de curry Args
   (list (car Args)
      (list 'list
         (lit (cadr Args))
         (list 'cons ''job
            (list 'cons
               (list 'lit (list 'env (lit (car Args))))
               (lit (cddr Args)) ) ) ) ) )
</code></pre>

<p>When called, it returns a function-building function which may be applied to
some argument:

<pre><code>
: ((curry (X) (N) (* X N)) 3)
-> ((N) (job '((X . 3)) (* X N)))
</code></pre>

<p>or used as:

<pre><code>
: (((curry (X) (N) (* X N)) 3) 4)
-> 12
</code></pre>

<p>In other cases, you are free to choose a shorter and faster solution. If (as
in the example above) the curried argument is known to be immutable:

<pre><code>
(de curry Args
   (list
      (cadr Args)
      (list 'fill
         (lit (cons (car Args) (cddr Args)))
         (lit (cadr Args)) ) ) )
</code></pre>

<p>Then the function built above will just be:

<pre><code>
: ((curry (X) (N) (* X N)) 3)
-> ((X) (* X 3))
</code></pre>

<p>In that case, the "environment build-up" is reduced by a simple (lexical)
constant substitution with zero runtime overhead.

<p>Note that the actual <code><a href="refC.html#curry">curry</a></code>
function is simpler and more pragmatic. It combines both strategies (to use
<code>job</code>, or to substitute), deciding at runtime what kind of function
to build.


<p><hr>
<h2><a name="macros">Do you have macros?</a></h2>

<p>Yes, there is a macro mechanism in PicoLisp, to build and immediately execute
a list of expressions. But it is seldom used. Macros are a kludge. Most things
where you need macros in other Lisps are directly expressible as functions in
PicoLisp, which (as opposed to macros) can be applied, passed around, and
debugged.

<p>For example, Common Lisp's <code>DO*</code> macro, written as a function:

<pre><code>
(de do* "Args"
   (bind (mapcar car (car "Args"))
      (for "A" (car "Args")
         (set (car "A") (eval (cadr "A"))) )
      (until (eval (caadr "Args"))
         (run (cddr "Args"))
         (for "A" (car "Args")
            (and (cddr "A") (set (car "A") (run @))) ) )
      (run (cdadr "Args")) ) )
</code></pre>


<p><hr>
<h2><a name="strings">Why are there no strings?</a></h2>

<p>Because PicoLisp has something better: <a
href="ref.html#transient-io">Transient symbols</a>. They look and behave like
strings in any respect, but are nevertheless true symbols, with a value and a
property list.

<p>This leads to interesting opportunities. The value, for example, can point to
other data that represent the string's translation. This is used extensively for
localization. When a program calls

<pre><code>
   (prinl "Good morning!")
</code></pre>

<p>then changing the value of the symbol <code>"Good morning!"</code> to its
translation will change the program's output at runtime.

<p>Transient symbols are also quite memory-conservative. As they are stored in
normal heap cells, no additional overhead for memory management is induced. The
cell holds the symbol's value in its CDR, and the tail in its CAR. If the string
is not longer than 7 bytes, it fits (on the 64-bit version) completely into the
tail, and a single cell suffices. Up to 15 bytes take up two cells, 23 bytes
three etc., so that long strings are not very efficient (needing twice the
memory on the average), but this disadvantage is made up by simplicity and
uniformity. And lots of extremely long strings are not the common case, as they
are split up anyway during processing, and stored as plain byte sequences in
external files and databases.

<p>Because transient symbols are temporarily interned (while <code><a
href="refL.html#load">load</a></code>ing the current source file), they are
shared within the same source and occupy that space only once, even if they
occur multiple times within the same file.


<p><hr>
<h2><a name="arrays">What about arrays?</a></h2>

<p>PicoLisp has no array or vector data type. Instead, lists must be used for
any type of sequentially arranged data.

<p>We believe that arrays are usually overrated. Textbook wisdom tells that they
have a constant access time O(1) when the index is known. Many other operations
like splits or insertions are rather expensive. Access with a known (numeric)
index is not really typical for Lisp, and even then the advantage of an array is
significant only if it is relatively long. Holding lots of data in long arrays,
however, smells quite like a program design error, and we suspect that often
more structured representations like trees or interconnected objects would be
better.

<p>In practice, most arrays are rather short, or the program can be designed in
such a way that long arrays (or at least an indexed access) are avoided.

<p>Using lists, on the other hand, has advantages. We have so many concerted
functions that uniformly operate on lists. There is no separate data type that
has to be handled by the interpreter, garbage collector, I/O, database and so
on. Lists can be made circular. And lists don't cause memory fragmentation.


<p><hr>
<h2><a name="floats">How to do floating point arithmetics?</a></h2>

<p>PicoLisp does not support real floating point numbers. You can do all kinds
of floating point calculations by calling existing library functions via
<code><a href="refN.html#native">native</a></code>, inline-C code, and/or by
loading the "@lib/math.l" library.

<p>But PicoLisp has something even (arguably) better: Scaled <a
href="ref.html#num-io">fixpoint numbers</a>, with unlimited precision.

<p>The reasons for this design decision are manifold. Floating point numbers
smack of imperfection, they don't give "exact" results, have limited precision
and range, and require an extra data type. It is hard to understand what really
goes on (How many digits of precision do we have today? Are perhaps 10-byte
floats used for intermediate results? How does rounding behave?).

<p>For fixpoint support, the system must handle just integer arithmetics, I/O
and string conversions. The rest is under programmer's control and
responsibility (the essence of PicoLisp).

<p>Carefully scaled fixpoint calculations can do anything floating points can
do.


<p><hr>
<h2><a name="bind">What happens when I locally bind a symbol which has a function definition?</a></h2>

<p>That's not a good idea. The next time that function gets executed within the
dynamic context the system may crash. Therefore we have a convention to use an
upper case first letter for locally bound symbols:

<pre><code>
(de findCar (Car List)
   (when (member Car (cdr List))
      (list Car (car List)) ) )
</code></pre>

;-)


<p><hr>
<h2><a name="hardware">Would it make sense to build PicoLisp in hardware?</a></h2>

<p>At least it should be interesting. It would be a machine executing list
(tree) structures instead of linear instruction sequences. "Instruction
prefetch" would look down the CAR- and CDR-chains, and perhaps need only a
single cache for both data and instructions.

<p>Primitive functions like <code>set</code>, <code>val</code>, <code>if</code>
and <code>while</code>, which are written in <code>C</code> or assembly language
now, would be implemented in microcode. Plus a few I/O functions for hardware
access. <code>EVAL</code> itself would be a microcode subroutine.

<p>Only a single heap and a single stack is needed. They grow towards each
other, and cause garbage collection if they get too close. Heap compaction is
trivial due to the single cell size.

<p>There would be no assembly-language. The lowest level (above the hardware and
microcode levels) are s-expressions: The machine language is <i>Lisp</i>.


<p><hr>
<h2><a name="segfault">I get a segfault if I ...</a></h2>

<p>It is easy to produce a segfault in PicoLisp. Just set a symbol to a value
which is not a function, and call it:

<pre><code>
: (setq foo 1)
-> 1
: (foo)
Segmentation fault
</code></pre>

There is another <a href="ref.html#codePointer">example</a> in the <a
href="ref.html#ev">Evaluation</a> section of the reference manual.

<p>PicoLisp is a pragmatic language. It doesn't check at runtime for all
possible error conditions which won't occur during normal usage. Such errors are
usually detected quickly at the first test run, and checking for them after that
would just produce runtime overhead.

<p>Catching the segmentation violation and bus fault signals is also not a good
idea, because the Lisp heap is most probably be damaged afterwards, possibly
creating further havoc if execution continues.

<p>It is recommended to inspect the code periodically with <code><a
href="refL.html#lint">lint</a></code>. It will detect many potential errors.
And, most of these errors are avoided by following the PicoLisp <a
href="ref.html#conv">naming conventions</a>.


<p><hr>
<h2><a name="ask">Where can I ask questions?</a></h2>

<p>The best place is the <a
href="mailto:picolisp@software-lab.de?subject=Subscribe">PicoLisp Mailing
List</a> (see also <a
href="http://www.mail-archive.com/picolisp@software-lab.de/">The Mail
Archive</a> and <a
href="http://dir.gmane.org/gmane.lisp.picolisp.general">Gmane.org</a>), or the
IRC <a href="irc://irc.freenode.net/picolisp">#picolisp</a> channel on
FreeNode.net.

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